Summary

UPDATE 2020-08-15 new files
Please see https://www.thingiverse.com/thing:4570721

Improved bottom filament feed system. Changes to the spool - and holders to use 608RS bearings to allow for smoother feeding. Changed the top guide to better hold a bowden tube as a guide for the filament and added a mount for a filament sensor. I used the sensor from TH3D
I used a 5/16" wooden dowel for the spool shaft at fits well and is pressed into the bearings.
The spool hole is 9.2 mm. I printed two of the filament new bearing pieces and attached them with M4 bolts and hammer head t-nuts